(1) Field of the Invention
This invention relates to an electrode arrangement with plate-shaped electrodes, each of which is attached to a support, for heating electric crucibles used in glass melting processes.
(2) Technical Consideration and Prior Art
Direct and indirect heating methods are known for heating glass crucible furnaces. Indirect heating has found acceptance only with small crucibles and pot furnaces.
Direct heating is effected by means of electrodes dipping or extending into the glass melt. Direct heating relies on the ionic conducitivity of the glass melt, which conductivity depends greatly on the chemical composition of the glass, especially its alkali and alkaline earth content.
It is known from W. Trier, "Glasschmelzofen" [Glass Crucible Furnaces], Springer Publishers 1984 (pages 208-239) to utilize for heating purposes rod-shaped or plate-shaped electrodes in various designs and arrays. Customarily, two plate electrodes are mounted vertically on two facing walls of the crucible.
Of great importance in configuring the design of a direct heating furnace is a knowledge of the resistance of the heating path. In this connection, it must be taken into account that as current flows from one electrode to another, the flux path expands so that the current density can be very different at different locations between the electrodes. Consequently, determination of local current density distribution and of current resistance is a major problem.
Considered in the calculation for energy output is the resistance of the refractory material and the resistance of the glass melt. In case of external disturbances, a reduction in electric resistance of the refractory material can lead to excessive local power consumption in the crucible and thus to temperature increases and destruction of the refractory material. This phenomenon, known as thermal instability, can arise, for example, upon the following conditions: failure of electrode cooling, increases in current density, gradients of specific electrical resistance in the refractory material, and geometrical expansion of the region under consideration. Thermal instability in the refractory material is less dangerous than instability in glass melt since it can be prevented by increasing glass exchange between the electrodes. However, thermal instability in the refractory material does not permit to any appreciable extent an increase in melting efficiency per unit volume in existing or prior art electrode arrangements and constructions.
Furthermore, variations in structural arrangements are limited due to the amount of cooling power required for cooling the electrodes, which is in the range of 2-6 kW per electrode. Moreover, the dimensions of each electrode should be selected according to specific ratios.